Objectives: To study molecular correlates of diminished neurovirulence in Sabin Vaccine strains of polioviruses by comparing their structure and assembly with wild type polioviruses. Major findings: The nucleotide sequences of RNA from type 1 poliovirus vaccine and the parental neurovirulent strain differ by about 5%. The largest virion polypeptide, VP1, of the vaccine is structurally labile and differs slightly in size from the corresponding protein in the virulent strain. Proposed course: The significance of the alterations in attenuated type 1 polioviruses will be assessed by determining whether similar molecular lesions exist in the vaccine strains of antigenic types 2 and 3. The strains would lend support to the hypothesis that a casual relationship exits between alterations in capsid architecture and diminished neurovirulence. As a spin off of the above findings, the potential use of analysis of viral proteins as a marker for vaccine strains will be explored. Strains of poliovirus isolated from human vaccine-associated paralytic disease, provided by the FDA and the Center for Disease Control, will be examined by these techniques and the results correlated with other biological markers.